1. Field of the Invention
This invention concerns a magnetoresistance sensor used in magnetic recording devices, and in particular a magnetic sensor and thin film magnetic head utilizing the spin valve magnetoresistance effect.
2. Background Information
In recent years, magnetic sensors have been developed which consist of a sandwich-structure spin valve film formed by layering a pair of magnetic layers enclosing a nonmagnetic layer on a substrate, in order to reduce the saturation magnetic field and raise the magnetic field sensitivity in magnetic heads for reproduction. In a spin valve film, whereas the magnetization in one of the magnetic layers (the pinned layer) is fixed in the direction of the element height by the exchange-coupling magnetic field with the adjacent antiferromagnetic layer, the other magnetic layer (the free layer) is generally given a single-domain structure in the track width direction of the element by a hard bias method using the magnetic field of a permanent magnet, and the magnetization rotates freely under the action of an external magnetic field.
The larger the unidirectional anisotropic magnetic field due to the antiferromagnetic layer, the better the single-domain structure of the pinned layer, and the more securely its magnetization is fixed, the more secure the linear magnetic response to an external magnetic field, so that the magnetic characteristics of the magnetic sensor are improved. Hence in the past various antiferromagnetic materials have been proposed. And, it is known that the characteristics of the antiferromagnetic material change depending on the material of the base layer.
For example, in unexamined patent application Hei8-315326, a magnetoresistance effect head is disclosed in which, by employing a crystalline soft magnetic film which has high resistance and can improve alignment as the base layer of a magnetoresistance effect film, the relative magnetoresistive change and other characteristics can be improved. Further, in this patent application is described the provision of Ta or some other nonmagnetic metal film as the base in order to improve the crystallinity of the aforementioned crystalline soft magnetic film. Further, a magnetoresistance sensor described in unexamined patent application Hei8-213238 uses a Ta base layer in order to render uniform the crystal orientation of the magnetic free layer.
Moreover, in unexamined patent application Hei9-16915 is disclosed the fact that by using a two-layer film of a Ta film and an NiFe-system alloy film as the base layer in a spin valve magnetoresistance transducer, the crystallinity of the antiferromagnetic layer is improved, and the magnetization of the pinned layer is sufficiently fixed so that a linear magnetoresistive change can be obtained. Moreover, in unexamined patent application Hei6-325934 is described how, in a magnetoresistance effect element, by means of a base layer with a two-layer structure in which a second base film of Ta or similar is placed between a first base film of a material with an fcc (face-centered cubic) lattice and the substrate, the (111) orientation of the ferromagnetic film formed on top is improved, while at the same time the surface smoothness can be enhanced.
However, for a spin valve film of the prior art provided with a base layer comprising a Ta film, the unidirectional anisotropy magnetic field due to the antiferromagnetic layer is normally limited to between 200 and 1000 Oersteds (Oe), so that when applied to a magnetic head, if the operating temperature rises the magnetization direction of the pinned layer changes, the magnetic transducing characteristics become unstable, and reliability suffers.
There is the further problem that, if the free layer in a spin valve magnetoresistance sensor is made thin, the reproduction output can be raised, but if the thickness is reduced below a certain limit (approximately 30 to 40 Å), the (111) orientation of the free layer becomes inadequate, the ferromagnetic interaction between the free layer and pinned layer is increased, so that the reproduction output declines rather than increasing and the sensor becomes magnetically unstable. For this reason the thickness of the free layer is normally set in the range from approximately 50 to 100 Å; but in order to raise the reproduction output, it is desirable that the thickness of the free layer be decreased as much as possible while maintaining magnetic stability.